This invention relates to an idling speed feedback control method for internal combustion engines, and more particularly to a method of this kind which is intended to prevent a lag in the feedback control of the idling speed, at the start of the same control immediately following rapid deceleration of the engine, to thereby prevent engine stall.
In an internal combustion engine, the engine can easily stall due to a drop in the engine speed when the engine is operated in an idling condition at a low temperature of the engine cooling water. To eliminate such disadvantage, an engine speed feedback control method has been proposed, which comprises setting a desired idling speed in dependence upon the engine cooling water temperature, detecting the difference between the actual engine speed and the desired idling speed, and supplying supplementary air to the engine in a quantity corresponding to the detected difference so as to minimize the same difference, to thereby control the engine speed to the desired idling speed.
Further, to eliminate a lag in the idling speed feedback control at the start of the same control, a method has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 58-124052, which is adapted to supply the engine with supplementary air in a quantity required for maintaining the engine speed at or in the vicinity of the desired idling speed before the feedback control of the idling speed is started, when the engine speed decreases below a predetermined value higher than an upper limit value of the engine speed at which the feedback control of the idling speed should be started, while the engine is decelerating.
However, if the engine is rapidly decelerated with the throttle valve fully closed, i.e. if the engine speed decreases at a large rate, particularly when the engine temperature is low and at the same time the clutch is disengaged, the charging efficiency of intake air supplied to the engine drops, such that the engine is not supplied with a mixture in a quantity sufficient for producing required output torque, which can cause a sudden drop in the engine speed, often resulting in engine stall depending upon the magnitude of load applied on the engine. However, if a supplementary air quantity is previously increased by a predetermined increment and the increased supplementary air is supplied to the engine before the start of the feedback control, in an attempt to avoid the above disadvantage, a further disadvantage can occur when the engine is slowly decelerated, i.e. if the engine speed decreases at a small rate, such that the engine speed cannot be promptly controlled to the desired idling speed, resulting in a lag in the feedback control of the idling speed.